I agree about the Notification Reminder in Accessibility - it works, but it's not the same tone (it's the default notification tone on your device, so I then tend to ignore it) and a bit inconsistent.
I am using Textra now and it has a setting for repeating notifications, up to 50 times (every two minutes). Works like a charm. Plus Textra has so many other options that make it so much better than the default app.